Monoclonal antibodies produced by hybridoma lymphocytes can have high specificity for cell membrane antigens present on distinctive cell types. Hybridoma technology has been employed in our laboratory to produce antibodies which bind selectively to hematopoietic progenitor cells. Because these cells are unavailable in quantities sufficient for immunization, the K562 leukemia cell has been employed as an antigen because it has properties similar to those of the true hematopoietic stem cell. Antibodies raised in mice against K562 cells show a variety of binding patters to normal cells, as assayed by various immunological methods. A small number of these antibodies are inhibitory to hematopoietic colony formation. At least one antibody has the capacity to bind to the myeloid progenitor, as demonstrated by fluoarescent activated cell sorting. Two anti-K562 monoclonal antibodies have been extensively characterized by immunochemical and biological means. These antibodies recognize antigens characteristic of the un-differentiated state of myeloid cells.